16 free guides
OBD2 Diagnostic Guides
Working method, live data, multimeter tests and the mistakes that waste money. They complement each code page and explain the reason behind every step.
Getting started
What the engine light means, how to read a code and which scan tool to use.
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Check engine light: what it means and what to do if it is steady or flashing
The difference between a steady and a flashing light, when you can keep driving, the most common causes and the steps to read the code without oversp…
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How to read DTC codes with an OBD2 scan tool, step by step
Where the connector is, what to save before you clear anything, and how to interpret several codes at once without getting the priority wrong.
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How to choose an OBD2 scanner: ELM327 adapter, code reader or multi-system scan tool
What each type of scanner can read, why many cheap adapters fail, and which features you need depending on whether you work on your own car or in a s…
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Generic and manufacturer-specific codes: P0, P1, P2, P3, B, C and U
How to tell whether a code means the same on every make or depends on the manufacturer, and what to do when your scanner shows a manufacturer-specifi…
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OBD2 glossary: car diagnostic terms explained
What DTC, MIL, freeze frame, STFT, PID, closed loop, bank 1 and other acronyms that appear on the scan tool and in code pages mean.
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Finding the fault
Live data, fuel trims, multimeter tests and the mistakes that waste money.
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Six mistakes that waste money when diagnosing a trouble code
The code does not name the broken part. These are the reasoning mistakes that lead to the most unnecessary parts being bought.
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How to test a sensor with a multimeter: power, ground and signal
A three-step method to find out whether the fault is the sensor, the wiring or the computer before you buy anything.
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Fuel trims (STFT and LTFT): how to read them to find the fault
What short and long-term fuel trims are, which values are normal, and how to use them to tell a vacuum leak from a fuel pressure problem.
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OBD2 live data: which parameters to watch and their normal values
The ten parameters a technician checks before replacing a part, with typical ranges for a warm gasoline engine and what each deviation points to.
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Electronic throttle: limp mode, cleaning and relearn
Why the engine suddenly stops accelerating, which codes appear, how to clean the throttle body without damaging it and how to run the idle relearn.
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Vacuum leaks: how to find them without replacing parts
Vacuum leaks are the number one cause of a lean mixture and a rough idle. How to confirm one with live data, and four ways to find it.
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After the repair
Clearing codes, completing the drive cycle and passing the inspection.
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OBD2 drive cycle: what readiness monitors are and how to set them to ready
Why your car can fail the inspection even with the light off, and how to complete the drive cycle that validates each system.
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How to clear DTC codes, and when you should not
Clearing a code turns the light off but repairs nothing. What you lose when you clear, why disconnecting the battery is a bad idea and when you shoul…
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How to prepare for an OBD2 emissions inspection: monitors and codes
What the inspection equipment checks through the OBD2 port, why you can fail with the light off, and how to get the monitors to ready before your app…
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Diesel engines
Particulate filter, regeneration and the AdBlue/SCR system.
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Diesel particulate filter (DPF): regeneration, codes and how to keep it from clogging
What regeneration is, why short trips clog the filter, what to do when the warning light comes on and when it makes sense to clean or replace it.
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AdBlue (DEF) and the SCR system: codes, the no-start countdown and how to avoid failures
How the SCR system with AdBlue (DEF) works, why a countdown appears that can stop the engine from starting, which codes are common and the care that …
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